ZOOLOGY. 867 
iiarly distinct within the thorax; each ganglion is connected 
to the next by a double cord, as in the ganglionic chain of 
inferior animals, and frequently gives off numerous twigs to 
the neighbouring vessels and viscera, as well as a constant 
one to the nearest spinal nerve. This chain of ganglia is 
most remarkable in the neck, where it is lodged in the canal 
formed by the transverse processes of the cervical vertebrae 
on each side. Superiorly, however, it terminates at the ex¬ 
tremity of this canal in the third cervical vertebra in a slender 
fibril, which bends outwards, and is connected with the vagus 
nerve, and also, according to Cuvier, with the fifth and sixth 
pairs. 
54. Brain of the Casoar, side view.—No. 23, the accessory 
nerve of Willis, and eighth pair; 4, the sixth pair; 5, the 
fifth pair; 6, fourth pair; 7, optic lobes; 8, third pair; 9, 
optic tubercles; 10, optic nerves; 11, pedicle of the hemis¬ 
pheres; 12, olfactory lobule; 13, lateral part of the cerebral 
hemispheres ; 14, lateral part of the cerebellum. 
53. Optic lobes and nerves of the ostrich.—No. 1, me¬ 
dulla spinalis; 3, interlacement of the fibres of the cerebral 
peduncles; 2, 6, optic lobes; 4,5, interlacement of the optic 
nerves. 
In the class mammalia the spinal marrow is no longer 
comparable to the brain in point of size, as in fishes and am¬ 
phibia ; nor in respect to the perfect developement of indivi¬ 
dual portions of it, as in birds; and consequently we find the 
spinal marrow gradually becomes more completely subordi¬ 
nate to the brain. The latter not only advances considerably, 
as relates to the perfection of its internal form, but also ac¬ 
quires a notable increase of bulk, as compared with the body 
in general, although the preceding class already, to a certain 
extent, approaches in that respect to the present. 
The spinal marrow, in general, is much more similar to 
that of man, as regards its form, position, and investments, 
than in the preceding classes, although there are sufficiently 
striking marks of distinction. One of the most essential is 
the canal of the spinal marrow, which probably exists in the 
whole class. The spinal marrow also extends lower down in 
the vertebral canal than in man ; and although it always 
forms a cauda equina (the origins of the last nerves sent off 
being higher up than the inter-vertebral holes), it still reaches 
into the sacrum, and even gives off nerves there, which pass 
through the holes of the caudal vertebrae, the cord itself no 
longer extending so far, except, perhaps, in the aquatic mam¬ 
malia. In this class, likewise, the triple swelling of the spinal 
marrow above the medulla oblongata, in the middle for the 
nerves of the anterior extremities, and below for those of the 
posterior extremities, is recogniiable, the last of the three 
being, in general, proportionally thicker than in man. It is 
remarkable that, in certain short-necked animals, as rats and 
mice, the upper and middle enlargements of the chord are so 
completely united into one mass, that the portion of the 
spinal marrow contained in the cervical vertebras is nearly as 
large again as the remaining part. The posterior groove of 
the spinal marrow in this class, and even in man, is less dis¬ 
tinct than in those preceding: it still exists, however, and is 
sometimes of considerable depth; for instance, in the ro- 
dentia and bats. The progressive improvements in the struc¬ 
ture of the brain in this class are very essential: they consist 
partly in the greater abundance of fibrous substance in the 
hemispheres, which are connected by an additional large 
commissure, and partly in the higher degree of developement 
of the cerebellum. The optic tubercles, on the contrary, 
recede, and have appended to them a second pair of ganglia, 
from which circumstance the whole mass receives the name 
of corpora quadrigemina. Speaking generally, the form of 
the brain, as it presents itself in the rodentia, constitutes the 
most perfect intermedium between the formation of the same 
organ as it exists in this and the preceding class. Of-the in¬ 
dividual masses composing the brain, the first, as already 
noticed, is peculiarly distinguished by the appearance of the 
corpus callosum and fornix, which, together with the ante¬ 
rior commissure, already existing in fishes, unite the two he¬ 
mispheres. The external form of the hemispheres in the ro- 
Vol. XXIV. No. 1678. 
dentia, as well as in the shrews, moles, and bats, is an oval, 
smallest anteriorly, and the upper surface is perfectly smooth, 
as in birds. Posteriorly, they do not cover the cerebellum, 
and frequently not the optic tubercles. Internally, the ante¬ 
rior fold or inflexion of the corpus callOsum, and its pro¬ 
longations into the lateral ventricles, (cornua ammonis) are 
particularly wide and large. The anterior pillars of the for¬ 
nix are very short, being soon lost behind the inflexion of 
the corpus callosum. The formation of the hemispheres in 
mammalia generally coincides, in a remarkable manner, with 
that of the optic tubercles in many of the osseous fishes, 
whence, in the latter, those tubercles have frequently, but in¬ 
correctly, been viewed in the light of hemispheres. The he¬ 
mispheres as they exist in the rodentia and chiroptera come 
next to those of birds: they do not present any fossae, or con¬ 
volutions, in the mouse, rat, marmot, beaver, bat, two-toed 
ant-eater, &c. In the carnivora, ruminantia, &c., they are 
much more voluminous, are furnished with distinct convolu¬ 
tions, and cover not only the whole of the corpora quadri¬ 
gemina, but also a part of the anterior surface of the cerebel¬ 
lum, in the same manner as in the foetus of the sixth and 
seventh month. In apes and porpusses the hemispheres are 
still larger, and here first present a third or posterior lobe, the 
imperfection of which is marked by the absence of convolu¬ 
tions upon it in most quadrumana, in the same manner as 
they appear there at the latest period only of the existence of 
the human foetus. M. Spurzheim, it is true, asserts that the 
third or posterior lobe of the hemispheres exists much more 
uniformly; in which respect he differs from Cuvier, Tiede- 
mann, and Serres. His opinion, too, appears to be contra¬ 
dicted by the fact, that the posterior horn of the lateral ven¬ 
tricle, and the hippocampus minor contained within it, first 
present themselves in the quadrumana. The corpora striata 
are present in mammalia, and in the rodentia and edenfata 
are much larger in proportion to the hemispheres than in car¬ 
nivora and ruminantia; thus forming an additional point of 
approximation to the organization of birds. Of the corpus 
callosum it is sufficient to remark, that its size and develope¬ 
ment have an immediate relation with the condition of the 
hemispheres. Such, also, is the case with the fornix, which 
is, in all instances, formed by a medullary layer, descending 
from the optic thalamus into the mammillary body on each 
side; from whence, re-ascending behind the anterior commis¬ 
sure to meet its fellow, it sends off a medullary lamina on 
each side to the under surface of the corpus callosum, called 
septum lucidum; and then again diverges from its fellow 
to form the cornu ammonis, and terminate on the outer sur¬ 
face of the hemisphere. 
Of the different portions of the third mass, the cerebellum 
in mammalia, is principally distinguished from that of the 
preceding classes, in this respect, that it is divided into a 
central mass and two lateral lobes. The middle or vermiform 
mass, so small in mat),, is usually very large in mammalia, 
apes alone, and, according to Cuvier, the porpoise excepted. 
The number of the laminae or transverse folds of the cere¬ 
bellum is generally much greater than in the foregoing 
classes; but also much less than in man: hence, also, these 
laminae are proportionally thicker and more convoluted,'so 
much so, that the vermiform process is sometimes almost S 
shaped. The bulk of the cerebellum is likewise greater in 
proportion to the rest of brain than in man. The remark- 
able increase of size and developement of the cerebellum in 
this class, seems to be closely connected with the appearance 
of the pons varolii, which was deficient in the preceding 
classes, and which may probably be correctly designated as 
the inferior commissure of the cerebellum. It coincides 
with such a view, that this part, which in man attains a very 
considerable size, is here generally, and in the rodentia par¬ 
ticularly, narrow. The pons is generally separated into a 
posterior and anterior portion. The anterior appears very 
distinctly in the mouse and bats, as the inferior commissure 
of the posterior pair of corpora quadrigemina; the posterior 
on the contrary, is often so little elevated, that the corpora 
pyramidalia, which are always very distinct in mammalia, 
9 G take 
